The invention relates to a propulsion control system for motor vehicles as shown in U.S. Pat. No. 4,484,280. The system of U.S. Pat. No. 4,484,280 reduces the power of the vehicle engine when a driven wheel tends to spin. Spin is determined when vehicle speed exceeds an upper vehicle speed threshold value during straight-ahead driving or a lower vehicle speed threshold value during cornering or when an indicated threshold value of the longitudinal acceleration of the vehicle is exceeded.
Unstable driving conditions occur, for example, when the driven wheels spin or when the maximally transferable concerning force is exceeded. To indicate in each case only one certain vehicle speed threshold value for straight-ahead driving and for driving turns, as in the case of the known system, meets the existing requirements only with respect to one respective operating point. In certain situations, an unstable condition may occur at speeds below these threshold values so that this system offers no suitable protection in this respect.
It is the objective of the invention to provide a system that is able to avoid unstable driving conditions when wheel slip occurs.
It is possible that unstable conditions may also occur when no wheel slip is measured, for example, when at a constant speed, an increasingly narrower turn is driven. Such conditions are measured by means of the comparison of the measured lateral acceleration with the threshold values derived from the difference of the wheel speeds and/or from the steering angle and are not the object of the invention.
The invention utilizes the longitudinal acceleration of the vehicle at the very moment at which a driven wheel tries to spin. This indicates that the adhesion coefficient is fully used, at least at one wheel. There is a direct relationship between the adhesion coefficient and the maximal longitudinal acceleration. A vehicle with all-wheel drive comes closest to the actual situation, all its wheels being in the wheel slip control range. In the case of rear-driven vehicles, the load will produce inaccuracy that, if necessary, can be compensated by a load-dependent emitter signal.
The maximal longitudinal acceleration and the maximal lateral acceleration have a relationship to one another via the so-called frictional connection cycle. The maximal lateral acceleration, in the case of all vehicle-road conditions, has approximately the same relationship to the maximal longitudinal acceleration and can either be calculated from it by multiplication or can be taken from an empirically determined table. Therefore, if a limit value is provided for the lateral acceleration, it is easy to indicate from it, in connection with the steering agle, a limit value for the vehicle speed. The actual vehicle speed will then be compared with the limit valve and reduced in the case of an exceeding of the driving power.
The value of the maximally possible longitudinal acceleration of the vehicle determined in the case of a slip tendency is therefore also an approximate value for the adhesion coefficient between the vehicle and the road at that moment. After a suitable conversion, this value can be displayed and can be used by the driver as a guide for his method of driving.
As a result, it is possible to reduce the power of the vehicle before slip-caused unstable driving conditions can occur.
A proposed control system to achieve these functions includes an electronic device for receiving the output signals of the steering angle sensor and, starting at the point in which the spinning tendency of at least one of the spin wheels is determined by the wheel slip control system, for receiving the output signal of a longitudinal acceleration sensor. The electronic device includes a first circuit for determining a value of lateral acceleration from the value of longitudinal acceleration that is determine when a spinning tendency of wheel occurs. A second circuit in the electronic device determines the threshold value of vehicle speed from the determined lateral acceleration value and the steering angle. This threshold value of vehicle speed is compared in a comparator with the output signal of the vehicle speed sensor and provides an output signal when the vehicle speed exceeds the threshold value. A control system for the power control element of the vehicle reduces the power in response to the output of the comparator. This reduction is irrespective of the position of the gas pedal and overrides the gas pedal. The first and second circuits may be calculators or include look-up tables. The input signal of the electronic device that is assigned to the longitudinal acceleration is provided to a display unit for displaying of value symbolizing a certain adhesion coefficient as a function of longitudinal acceleration input. The display may be acoustic and/or visual.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.